This film is not based on a real life story. It is, however, based on a real life ad, which ran in the classified pages of Backwoods Home Magazine in 1997. It read: ‘Wanted: Somebody to go back in time with me. This is not a joke. You’ll get paid after we get back. Must bring your own weapons. I have only done this once before. Safety not guaranteed.’

While everyone else spent a decade speculating about the ad’s origin, writer Derek Connolly and director Colin Trevorrow have gone a step further; inventing a backstory that pieces the mystery together. In doing so, they’ve pulled off something almost as impossible as time travel – a lo-fi indie romance with a deadpan wit that is never glib, and a touching sweetness that is never cloying.

Aubrey Plaza is Darius, a despondent magazine intern with a face like Wednesday Addams and the sardonic manner of MTV’s cult cartoon teenager Daria. Together with her jackass superior, Jeff (Jake Johnson), and geeky fellow intern Arnau (Karan Soni), Darius is sent to track down the mooncalf who posted the ad. And so they discover Kenneth (mumblecore vet Mark Duplass), an eccentric shelf-stacker who claims that his time machine is real and that government agents are following his every move.

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While Darius plays along for the sake of the feature, Jeff uses the work holiday as an excuse to look up an old girlfriend who he knew in the days before he became a shallow and cynical hack. Having set up these stereotypes, Connolly’s script proceeds to do delightful, unexpected things with them, allowing his characters to break out of their conventional packaging and become real, complex people.

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The narrative flirts with different genres, defying any assumptions you might make about what happens next. Familiar tropes are playfully subverted: a stealth break-in to a high-tech science lab is interrupted by a hilariously mundane office party; a climactic car chase take place at 15mph.

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In the hands of a less talented cast this delicate and witty construction could have easily fallen to pieces. But the performances are impeccable, combining dry comic timing with a rich emotional vulnerability that makes the film a surprising joy.

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Trevorrow is extremely assured for a debut fillmaker. Though Safety Not Guaranteed is rooted in a suburban setting, he’s created a fantastical world full of the wet undergrowth of childhood games and the bright, misty mornings you stayed up all night to discover as an adult.

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There’s a wistful longing at the heart of the film. Each of the characters wants to go back to a time of innocence that they feel they’ve lost. “You can’t just go and find that stuff again,” one character argues. ‘So what if you can’t go back?’ the film replies. Why would you want to when you could be having this adventure right now?